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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 30 January 2017 by dana1981

It is extremely likely [95 percent confidence] more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together … The best estimate of the human-induced contribution to warming is similar to the observed warming over this period … The contribution from natural forcings is likely to be in the range of −0.1°C to 0.1°C, and from internal variability is likely to be in the range of −0.1°C to 0.1°C.

In fact, the report’s best estimate was that humans are responsible for all of the global warming since 1951, and greenhouse gases for about 140%. That’s because natural factors have had roughly zero net effect on temperatures during that time, and other human pollutants have had a significant cooling effect.

In other words, the Earth’s surface warmed about 0.65°C between 1951 and 2010. Human greenhouse gas emissions caused temperatures to be about 0.9°C hotter than they would have otherwise been. But other human pollutants caused about 0.25°C cooling, and natural factors had a very small effect.

This expert conclusion is quite different from the comments of Trump’s cabinet nominees, who were all very consistent in admitting that there is “some connectivity,” as Trump put it, between human activity and global warming, but claimed that the degree of human influence remains open for debate.

Here’s the evidence

The IPCC conclusion on human-caused global warming rests upon a broad and deep base of evidence. To start, there’s basic math and physics. Long-term global temperature changes are a response to changes in the Earth’s energy balance. If there’s more incoming than outgoing energy, temperatures rise. For example, that happens when the sun becomes more active (more incoming energy), or when the greenhouse effect increases (trapping heat, decreasing outgoing energy).

The increase in heat trapped below the atmosphere and decrease in heat escaping to space are also two of the many “fingerprints” of human-caused global warming. These are changes scientists expect to see if human carbon pollution is the culprit behind global warming.

As another example, we expect to see nights warming faster than days, because at night there’s a smaller volume of air, which allows the increased greenhouse effect to have a bigger effect on temperatures. Warming due to the sun would likely have the opposite effect, causing faster daytime temperature rise when sunlight is bombarding the Earth. For all of these “fingerprints,” observations are consistent with human-caused global warming and many are simply inconsistent with natural warming.

There have also been numerous studies specifically investigating and quantifying the various contributors to global warming. I summarized ten of these in the graphic below, and provided details about each study here. The results have been consistent with the IPCC conclusion – humans are responsible for essentially all of the global warming since 1950.

Scientific consensus arises from consilience of evidence

There is of course a 97% expert consensus on human-caused global warming. That consensus isn’t a result of groupthink, or a vast conspiracy, or money-grubbing scientists lying to get their greedy little hands on grant funds. In fact, scientists are very hard to convince of anything, and they’d much rather be the one to disprove a theory than the twenty-thousandth to reaffirm it.

But scientists base their conclusions on evidence, and as discussed above, the evidence for human-caused global warming is overwhelming. In the video below from the Denial101x free online course, Peter Jacobs discusses the three pillars of this type of knowledge-based consensus.

Denial 101x knowledge-based consensus lecture by Peter Jacobs.

Verdict: Trump and his cabinet picks are right that humans are influencing global warming, but they're wrong to doubt that humans are the dominant cause of the warming over the past six decades.

Comments

The outlook for the Paris Agreement and the funding of climate scientists doesn't look too good at the moment, if media reports are to be believed. It seems that the new administration may not be entirely convinced by the 97% consensus that humans are responsible for all of the global warming since 1950. If so what steps will/can climate scientists, particularly in the US but also in other locations, take to convince the new administration that climate change is dangerous? Presumably plans were put in place before Mr Trump was elected as it has been known for many months that his election was a possibility. Have any plans been made? If so will they be made public? If not will plans be made or will climate scentists vacate the field as it were?

Apologies. Having written comment 1, I then scrolled down the page to see, much to my chagrin, the next item was "March for Science'. Good-o but as the 45th POTUS has dismissed other such marches do you thinkthere will be on going and varied protests? This is political I guess rather than science so if it is scrubbed then fair enough

Secretary of State Rex Tillerson is in an invidious position - he is from an engineering background, and he has sat on top of Exxon Mobil's extensive scientific work on climate change.

Tillerson would have been nobody's ideal choice as Secretary of State, but it will be interesting to see how he handles climate diplomacy. The US has already handed Asian market penetration over to China in a whole range of goods (by ditching TPP), now it may concede global climate leadership to China also. In this video by Peter Sinclair, Dan Kammen reflects on Tillerson's options.

I often hoped the same as this and even went on record that this might playout untill I saw Sonny Perdue ended up being the sec ag nominee. If we had a more forward thinking sec of ag then the two working together might have given us a huge advantage. We have enough agricultural land that this pricing of externalities could have brought us either to a net negative carbon or very low indeed. We even have a similar blueprint (though voluntary) here in OK already set up. Alas it may be just a pipe dream.